The use of optics to provide security checks and to validate color documents such as paper currency are known. Optical sensor systems typically comprise one or more light sources, a light detector, interface circuitry and a discriminator or microprocessor. The sensors measure the amount of light transmitted through or reflected from documents and an analog-to-digital converter outputs the digital data to a microprocessor for processing. Some prior art optical detection systems use an analog-to-digital converter to convert sampled optical signals into one of 256 possible values for processing. Measurements are typically taken over many predetermined places of the document under test.
U.S. Pat. No. 4,947,441 describes a bill discriminating apparatus having two color detectors for photoelectrically detecting light components contained in light transmitted through or reflected from bills to be discriminated. The bill discriminating circuitry includes a color correction circuit, two amplifying means, a gain adjustment circuit, and a differential amplifier to compare the outputs of the color detectors. A discriminator determines the validity of bills based on difference signals provided by an output from the differential amplifier.
A problem plaguing such prior art systems is that the electrical signal produced by the optical detectors typically comprise offsets which skew the bill data that is to be processed. The electrical offsets may include offsets resulting from manufacturing of the sensor, production of the optical system, and ambient light. Another electrical offset may be caused by an increase in temperature which produces a decrease in the illumination output of an LED light source. The corresponding decrease in detected light further skews the bill data.
These electrical offsets may be present in any combination and may vary from sensor to sensor. The combined electrical offsets make it virtually impossible for a microprocessor to discriminate actual bill data from the offsets. As a consequence, accurate ratio testing between two or more sensor outputs required by particular known acceptance algorithms is very difficult.
Several known calibration techniques are able to remove specific types of electrical offsets. For instance, one calibration technique utilizes additional hardware to regulate the current through LED light sources for adjusting their light output to balance the outputs of the corresponding optical sensors. This technique minimizes the electrical offsets due to manufacturing of the sensor and production of the system, but is costly to implement. Another well known calibration technique relies on the microprocessor to apply a stored compensation factor to the digital data obtained from the analog-to-digital converter to minimize the effects of the electrical offsets. However, this technique suffers from poor data resolution due to the inclusion of all of the electrical offsets in the data signal.
Therefore, a need exists for a low cost optical sensor system having an optical interface circuit that minimizes the effects of electrical offsets to achieve a maximum data resolution.